A sentence is in active voice when the subject is the doer of the action.
A sentence is in passive voice when the new subject (object in active voice) is the receiver of that action.
The
object in the active voice now takes the place of the subject, and vice
versa. (subject + verb + object, where the subject and object swap
places)
The structure of the given sentence is: Subject (The news of ceasefire) +
verb (pleased) + object (the soldiers).
When this is changed to passive voice, the correct structure must be:
New subject (the soldiers) + verb (were pleased) + by + new object (the news of ceasefire).
The
preposition 'by' is essential in passive voice, so that the meaning of
the sentence does not change, the original subject 'the news of ceasefire' still
performs the action. Also, 'pleased' is changed to 'were pleased', as the voice changes.
Option B: The structure mentioned above can be seen in B, hence it's the correct option.
Option
A is incorrect because the subject has been altered from a phrase 'the news of ceasefire' to a word 'ceasefire'.
Option C is incorrect as the preposition
'by' is missing. The sentence is hence, incomplete in meaning.
Option D is incorrect as the correct answer only lies in B.